1. Field of the Invention
The present invention relates to an image pickup apparatus that enables shooting with a light emission device (a flash device), and a control method therefor.
2. Description of the Related Art
A wireless stroboscope system that establishes a wireless connection between a master flash device connected to the camera and a slave flash device using optical communications by lights from discharge tubes to emit lights synchronously is known conventionally. For example, Japanese Laid-Open Patent Publication (Kokai) No. 2006-84608 (JP 2006-84608A) discloses a flash device that is mounted on a camera providing a focal plane shutter to construct a wireless stroboscope system.
Since the optical communications of the conventional wireless stroboscope system are one-way communications from the master flash device to the slave flash device, the master flash device is not able to receive a reception confirmation signal from the slave flash device. Therefore, the following methods are adopted to check whether communication between the master flash device and the slave flash device is established. In one method, a test emission signal is transmitted from the master flash device, and when receiving the signal, the slave flash device performs test emission to inform that the communication is established. In another method, a user checks whether the slave flash device emits a light at the time of shooting. In such a checking method, since the master flash device and the slave flash device have to emit light once, there are problems of wasting energy and of requiring a time for re-charging.
In order to solve the problems, a wireless stroboscope system that enables two-way communications between the master flash device and the slave flash device using radio waves can be designed. However, when the wireless stroboscope system using radio waves is used in combination with a camera having a mechanical shutter such as a focal plane shutter, an operation of the mechanical shutter needs to start after the establishment of communication is confirmed at the time of a light emission start. Therefore, the following problem of time lag occurs.
That is, when a release switch is pushed and a control unit of the camera starts a process of a shooting operation, the control unit of the camera controls the master flash device to transmit a transmission checking signal to the slave flash device. When receiving this signal, the slave flash device replies a communication confirmation OK signal to the master flash device. When the master flash device receives the communication confirmation OK signal, the control unit of the camera starts to run a first curtain of a mechanical focal plane shutter. However, in this case, the first curtain of the mechanical focal plane shutter is driven after the control unit of the camera receives the communication confirmation OK signal via the master flash device. Therefore, there is a remarkable time lag between the timing at which the release switch is pushed and the timing at which the first curtain of the focal plane shutter starts to run.